1. Field of the Invention
The present invention relates to a planarizing method and apparatus thereof, in planarizing technique of a wafer surface pattern as made by polishing employed in the production process of a semiconductor device, to be achieved by using a grindstone as a polishing tool and an additive-containing processing liquid in order to attain polishing which features excellent flatness, excellent uniformity, high efficiency and large process margin.
2. Description of the Related Art
A semiconductor production process has a number of processing steps. Among them, a wiring step and a shallow trench isolation step which need wafer planarization done by polishing will be described with reference to FIGS. 2A to 5C.
First, a wiring step will be described. FIG. 2A is a cross-sectional view of a wafer having a first wiring layer formed thereon. Over the surface of a wafer substrate 1 on which a transistor is to be formed, an insulating film 2 is formed. A wiring layer 3 made of aluminum or the like is disposed over the insulating film. Since a hole is opened in the insulating film 2 to ensure connection with the transistor, the portion 3xe2x80x2 of the wiring layer is a little recessed. In a step of forming a second wiring layer as illustrated in FIG. 2B, an insulating film 4 and an aluminum layer 5 are formed over the first wiring layer, followed by deposition of a photoresist layer 6 for forming this aluminum layer into a wiring pattern. Then, as illustrated in FIG. 2C, the wiring pattern is transferred onto the photoresist layer 6 by exposure to light through a stepper 7. The concave and convex 8 on the surface of the photoresist layer 6 are not in focus simultaneously, which however depends on their step difference and in such a case, it causes serious disturbances such as defocus.
To overcome the above-described inconvenience, the wafer surface is planarized as will be described later. After forming the first wiring layer as illustrated in FIG. 3A, an insulating layer 4 is formed as illustrated in FIG. 3B and then, polishing is conducted to planarize the surface to the level of 9 in this diagram by the below-described method, whereby the state of FIG. 3C is obtained. A aluminum layer 5 and photoresist layer 6 are then overlaid, followed by exposure to light through a stepper as illustrated in FIG. 3D. The resist has a flat surface in this stage so that defocus as described above does not occur.
In the next place, the shallow trench isolation step will be described with reference to FIGS. 4A to 5C. This shallow trench isolation aims at insulation between elements on the substrate by embedding an insulating film in the shallow trench of the silicon substrate. FIG. 4A illustrates the deposition of a thin thermal oxide film 44 and a silicon nitride film 41 and then formation of a shallow trench 40 by dry etching both the upper films and the underlying silicon substrate. As illustrated in FIG. 4B, an insulating film 2 is embedded in the trench by CVD. Then, a photoresist layer 6 is disposed as illustrated in FIG. 4C. The photoresist 6 is left only at the trench portion as illustrated in FIG. 4D by lithography through a mask (reverse mask) obtained by negative-positive reversal of the mask used for formation of the shallow trench. With this photoresist 6 as a mask, dry etching is conducted to remove the insulating film 2 down to a predetermined depth 45, whereby the substrate as illustrated in FIG. 5A is obtained. Planarization polishing which will be described later is conducted to remove the insulating film 2 to the target level 9. The substrate becomes the state of FIG. 5B when polishing is conducted until complete removal of the insulating film 2 over the silicon nitride film 41. In FIG. 5B, the silicon nitride film 41 is thoroughly exposed and the insulating film 2 remains only in the shallow trench. By the subsequent steps, elements including transistor 42 are formed at the position from which the silicon nitride film 41 has been removed. With a view toward not deteriorating the characteristics of these elements, it is necessary to control the thickness of each of the silicon nitride film 41 and the insulating film 2 remaining in the shallow trench under markedly severe standards. To satisfy these standards, direct polishing is not conducted at the stage of FIG. 4B but steps FIG. 4C to FIG. 5A for relieving the polishing load are added.
A description will next be made of a planarization method employed for the above-described step. FIG. 6 illustrates CMP (chemical mechanical polishing) which is one of the most popularly employed methods. A polishing pad 11 adhered onto a platen 12 is turned. This polishing pad is, for example, polyurethane sliced into a thin sheet. A wafer 1 to be processed is fixed to a wafer holder 14 via an elastic backing pad 13. The convex portion of the insulating film 4 on the wafer surface is planarized by applying a load on the surface of the polishing pad 11 while turning this wafer holder 14 and feeding a polishing slurry 15, which is a processing liquid containing abrasive grains, onto the polishing pad 11.
As a processing technique superior to the above-described CMP in planarity, disclosed in Patent Application No. PCT/JP95/01814 is a planarizing technique using a grindstone. FIG. 1 illustrates a planarizing method by using a grindstone. An apparatus used for this technique is basically similar to that employed for CMP (chemical mechanical polishing) using the above-described polishing pad but it features that a grindstone 16 containing abrasive grains such as cerium dioxide is installed, instead of a polishing pad, onto a rotary platen 12. Planarization can be conducted only by supplying, as a processing liquid 18, abrasive-grain-free deionized water which corresponds to fumed silica in CMP. This method using a grindstone is excellent in planarizing the step difference of a pattern and is able to completely planarize a large-size pattern, for example, a pattern of several mm in width, which cannot be attained by the conventional method. Use of a grindstone high in a using efficiency of abrasive grains instead of an expensive polishing slurry low in a using efficiency of abrasive grains enables a cost reduction. With regards to scratches presumably caused by the adoption of a grindstone, it is possible to prevent even scratches, which are too small to be observed by naked eyes, by using abrasive grains one figure finer than those ordinarily employed for a grindstone. Described specifically, ultrafine abrasive grains having an average particle size of 0.2 to 0.3 xcexcm and a maximum particle size of 2 xcexcm, preferably 99% of which have a particle size of 1 xcexcm or less are used. By fine division of abrasive grains happens to lower a removal rate, but use of an additive as shown in Japanese Patent Unexamined Publication No. 2000-173955 makes it possible to positively release abrasive grains from a grindstone and improve a removal rate.
Problems of the above-described CMP method and the processing method using a grindstone will next be described in this order.
In the CMP (chemical mechanical polishing) method, the planarizing capacity is insufficient because a polishing pad has not a high modulus of elasticity. Since the polishing pad is brought into contact with not only the convex portion but also the concave portion and a load is applied thereon upon processing, the size of a pattern which can be planarized is several mm in width at maximum. It is very difficult to planarize patterns, such as DRAM, formed on a cm order. For the similar reason, even in a shallow trench isolation step, a soft polishing pad excessively polishes and removes an insulating film in the shallow trench and this phenomenon (dishing) deteriorates the characteristics of elements. As a countermeasure, ordinarily employed is a process of conducting lithography using a reverse mask and removing the convex portion by dry etching in advance, thereby lowering the polishing load. This countermeasure increases the number of the steps and in addition, planarity is not satisfactory yet. In addition, the polishing slurry used for CMP is accompanied with the problems that a special care must be taken because a polishing slurry tends to contain an acid or alkali; and abrasive grains in the slurry tend to scatter and induce an increase of particle residues in a clean room.
As a method for overcoming the above-described problems of CMP, proposed is processing using a grindstone instead of the polishing pad and slurry. The modulus of elasticity of the grindstone is about one figure higher than that of a polishing pad so that shortage of planarity can be overcome. By adoption of this method, a wide convex on the cm order can be planarized in a wiring step, while in an element isolation step, planarity is so high as to carry out processing with less dishing even if a reverse mask is not used. In addition, this method does not use slurry, which permits easy handling and suppression of an increase in the residual of scattered abrasive grains. When a grindstone is employed, on the other hand, the following problems will occur.
The processing method using a grindstone involves a problem in the trade-off between planarity and uniformity. In general, as the modulus of elasticity increases by changing a processing tool from a polishing pad to a grindstone, the higher the planarity of the convex, but the uniformity in the removing amount on the wafer deteriorates. This is because a processing tool having a high modulus of elasticity such as grindstone selectively removes the convex having a slight difference in pressure so that it is apt to be influenced by minute unevenness on the substrate or irregular pressure upon processing.
As described above, CMP and processing with a grindstone have not yet been free from the problems, that is, insufficient planarity and insufficient uniformity, respectively.
In addition, they have, as a common problem, a shortage in the process margin upon completion of the shallow trench isolation step. In the shallow trench isolation step as illustrated in FIGS. 4A to 4D or FIGS. 5A to 5C, it is necessary to control, with good precision, the remaining thickness of the nitride film 41 for protecting an active region and the remaining film thickness in the element isolating shallow trench 40, because these remaining films require very delicate treatment. When removal is not enough, the remaining insulating film over the nitride film or extra insulating film protruding from the shallow trench deteriorate the characteristics of the device. Excessive removal on the other hand damages the active area or causes insulation failure at the shallow trench isolation portion. Ideally, polishing is completed when the surface of the nitride film 40 is completely exposed at any point on the wafer. In reality, time ensuring a sufficient polishing amount all over the surface of the wafer is markedly short and the terminal point of polishing cannot be detected easily even by using some means. With a view to overcoming this problem, proposed in Japanese Patent Unexamined Application No. 9-208933 is a method using, in CMP, silicon nitride fine particles and an acid as a polishing agent, thereby increasing a removal rate selectivity ratio of a silicon dioxide film relative to a silicon nitride film. In this method, however, there still remains a problem that high planarity cannot be attained because of a low modulus of elasticity of the polishing pad. For example, in the shallow trench isolation step, even if a selectivity ratio is high, a soft polishing pad is brought into contact with the central portion of the shallow trench more strongly than the other portion so that excessive polishing of the film at the center of the shallow trench inevitably occurs, thereby forming a dishing. In addition, this method employs an acid so that some countermeasures against corrosion of a metal film, if any, or an apparatus are necessary.
The present invention is made to overcome the above-described problems by adding a dispersant to a processing liquid in a grindstone-using planarizing method of a wafer surface pattern.
There are some types of dispersants suitable for this planarizing method. A dispersant which intensively acts on the interface between a processing liquid and abrasive grains or a film to be processed such as a surfactant is most effective. Such a dispersant exists mainly on the interface between abrasive grains and a processing liquid, prevents agglomeration of abrasive grains owing to electric repulsive power or steric hindrance effect by its molecular shape, thereby heightening dispersity. The dispersant properly used in combination with a hydrophilic group or a hydrophobic group selectively adsorbs to a film to be processed and acts on the film. It concentrates on the interface so that even a trace amount of it is effective. Ordinarily employed surfactants tend to contain an alkali metal such as sodium and adversely affect a semiconductor device so that they are not suited. Neither are those containing a heavy metal as an impurity. In addition, dispersants which are safe and have less influence on the environment are desired, because they are used for mass production in a plant. As a dispersant which can satisfy the above-described limitations and can well disperse inorganic oxide fine particles serving as abrasive grains, dispersants made of a polycarboxylate can be mentioned as an example. Sodium salts of it are ordinarily employed, but ammonium salts are suited in consideration of the influence on a semiconductor. Among the polycarboxylates, ammonium polyacrylate is especially preferred.
When a grindstone having inorganic fine particles as raw material abrasive grains is employed, ammonium polyacrylate preferred as a dispersant is particularly effective. More specifically, when inorganic fine particles made of cerium dioxide, aluminum oxide, silica, zirconium oxide, manganese oxide, titanium oxide or magnesium oxide, or a mixture thereof are used as abrasive grains, ammonium polyacrylate has high dispersing effect, thereby bringing about a high removal rate and a high-quality surface. In addition, adhesion property of ammonium polyacrylate differs between a silicon nitride film and a silicon oxide film, resulting in an improvement in the removal selectivity ratio of two these films.
A dispersant having, as well as the above-described effects, abrasive grain dispersing effect or selective adsorbing property to a film to be processed is usable in the present invention. A dispersant capable of changing the surface potential of the abrasive grains generates repulsion between abrasive grains, thereby heightening the dispersity. At the same time, selective adsorption of a dispersant onto the surface of a film to be processed causes a change in the surface potential (zeta potential serves as an index) which varies, depends on the kind of the film, thereby bringing about a change in a removal selectivity ratio. Adjustment of the pH of the processing liquid to alkali is effective for causing the film surface and the surface of the abrasive grains to have a zeta potential of equal polarity, allowing repulsion to work and heightening the dispersity. Ammonia or an ammonium salt is preferably added for this purpose. Similarly, surfactants acting on the interface between the abrasive grains and processing liquid, thereby changing the zeta potential are effective as the dispersant of the present invention.
Polycarboxylates other than the above-described ammonium polyacrylate are also effective for dispersion of abrasives or selective adsorption to a film to be processed so that they are also suited as the dispersant of the present invention. Polycarboxylates such as acrylate and maleate are particularly effective as the dispersant. In addition to the polycarboxylates, dispersants such as polyoxyethylene derivatives, phosphate condensates, lignin sulfonates, aromatic sulfonate formalin condensates and alkylamines are also effective.
The above-exemplified dispersant has bring about marked effects when added to a grindstone made of finely divided abrasive grains for a semiconductor. It is particularly effective for a grindstone formed of abrasive grains which have an average particle size of 0.2 to 0.3 xcexcm and at least 99% of which have a maximum particle size of 1 xcexcm or less.
The particle size of the abrasive grains is very fine so it is measured by laser light scattering method or electron microscopy. Laser light scattering method capable of measuring a number of particles has less diversity but measuring error appears when nonspherical particles or weakly agglomerated particles are measured. Measurement by electron microscopy, on the other hand, can correct errors due to particle shape or agglomeration, but has diversity because the number of particles which can be measured is small. The abrasive grains effective in the present invention has a particle size, as an average of the particles measured, of 0.1 to 0.4 xcexcm and at least 99% of the particles have a particle size of 1 xcexcm or less. The size of the particles in a nonspherical form is expressed by (the longest diameter+the shortest diameter)/2. Upon laser light scattering method, agglomeration happens to appear owing to the pretreatment of a measuring instrument or of a sample and in such a case, a large number of particles having a particle size of 1 xcexcm or greater seem to exist. This must be taken into consideration upon measurement. If no agglomeration due to pretreatment occurs, abrasive grains which have an average particle size, among the particles measured, of 0.2 to 0.3 xcexcm and at least 99.9% of which have a particle size of 1 xcexcm or less as a result of laser light scattering method are effective in the present invention.
Depending on the hardness of abrasive grains, kind of a film to be processed or a step, abrasive grains having a particle size outside the above-described range may be used. The abrasive grains having an average particle size, among particles measured, within a range of 0.05 to 0.5 xcexcm, preferably 0.1 to 0.4 xcexcm are effective. In terms of the maximum particle size (the maximum particle size of 99% or 99.9% of the particles), abrasive grains of 2 xcexcm or less, more preferably 1 xcexcm or less are preferred. There is no limitation on the minimum particle size in principle, but abrasive grains having a particle size of 0.001 xcexcm or greater is practically employed.